update your_github.md to say use any Vz email address, not just VMG email address

[imthenachoman]

https://github.com/VerizonMedia/oss-guide/blob/master/docs/resources/your_github.md

If this guide is truly meant for all of Verizon then, for the sake of clarity, step 3 should say something like "Add your Verizon email address to your account." instead of "Add your verizonmedia.com email address to your account."